In physics class, the girl to boy ratio is $3$ to $4$. If there are a total of $35$ students, how many boys are there?
Explanation: A ratio of $3$ girls to $4$ boys means that a set of $7$ students will have $3$ girls and $4$ boys. A class of $35$ students has $5$ sets of $7$ students. Because we know that there are $4$ boys in each set of $7$ students, the class must have $5$ groups of $4$ boys each. There is a total of $20$ boys in physics class.